This invention relates to heat shields and it is concerned particularly, but not exclusively, with heat shields for use in metal processing.
In steel mill processing whether of billets, strip or sections, the metallurgical qualities of finished product are closely related to the accurate control of temperature of the material during the hot rolling process. For example, a modern hot strip mill producing steel coil is several hundred meters long and typically, steel slabs or billets may be reduced from 25 cms thickness to 0.2 cms using several roughing mill stands and five or more finishing mill stands. During the rolling process considerable heat losses occur so that the slabs have to be heated initially well in excess of the temperature requirement at the end of the process, but a particular problem has been that the heat losses from slabs passing along the mill depend upon the time taken. If the slabs are delayed, excessive heat losses occur and the steel strip does not have its required rolling temperatures, so that it may have to be downgraded or even scrapped. In many long modern rolling mills the delay of one length of strip at the finishing end has an effect on the several lengths of material which are simultaneously at various preceding stages of rolling. Thus with more stringent quality specifications it is becoming more important to reduce the rate of heat loss from the material during transport between stands.
There is an added difficulty in this because during the final reduction stages the back end of the strip takes longer to pass through the finishing mills and so there is a temperature "run-down" along the steel strip due to the cumulative time delay along the length of the strip. The effect of temperature "run-down" is to some extent ameliorated by accelerating the finishing mills during the rolling of each individual slab or strip, but nevertheless it remains a problem.
Attempts have been made in the past to reduce the heat loss from the top surface of a hot strip during transport from the roughing mills to the finishing mills. Because radiation is a major source of heat loss at the temperatures involved (around 1060.degree. C.) aluminium reflectors have been fixed over the path of the hot strip to reduce temperature "run-down". However maintenance problems limit the usefulness of reflectors which become inefficient as soon as they become dirty, and in addition, the aluminium reflectors which have been used for their high reflectivity and relatively low cost can reach their melting temperature if their reflectivity decreases.
It has been proposed (UK Pat. No. 1,040,420) to use heat-insulating or reflecting panels as heat shields that present to the hot material a face formed by a thin plate of stainless steel backed by a core of thermal insulation, so that the plate forming the hot face of the panel is preheated to a luminous temperature close to that of the material being processed. In this arrangement, however, the panel could be easily damaged if there is a malfunction that causes material being processed to strike the panel, for example if the material is bent or if it lifts as it runs along its path. A mishap of this kind can easily occur in a steel rolling mill in which the steel stock is often travelling at very high speeds.